231 research outputs found
Superconductivity and Electronic Structure of Perovskite MgCNi3
The electronic structure, stability, electron phonon coupling and
superconductivity of the non-oxide perovskite MgCNi are studied using
density functional calculations. The band structure is dominated by a Ni
derived density of states peak just below the Fermi energy, which leads to a
moderate Stoner enhancement, placing MgCNi in the range where spin
fluctuations may noticeably affect transport, specific heat and
superconductivity, providing a mechanism for reconciling various measures of
the coupling . Strong electron phonon interactions are found for the
octahedral rotation mode and may exist for other bond angle bending modes. The
Fermi surface contains nearly cancelling hole and electron sheets that give
unusual behavior of transport quantities particularly the thermopower. The
results are discussed in relation to the superconductivity of MgCNi.Comment: 4 pages, RevTex, 5 ps figure
Structural and Magnetic Properties of Pyrochlore Solid Solutions (Y,Lu)2Ti2-x(Nb,Ta)xO7+/-y
The synthesis and characterization of the pyrochlore solid solutions,
Y2Ti2-xNbxO7-y, Lu2Ti2-xNbxO7-y, Y2Ti2-xTaxO7-y and Lu2TiTaO7-y (-0.4<y<0.5),
is described. Synthesis at 1600 C, and 10-5 Torr yields oxygen deficiency in
all systems. All compounds are found to be paramagnetic and semiconducting,
with the size of the local moments being less, in some cases substantially
less, than the expected value for the number of nominally unpaired electrons
present. Thermogravimetric analysis (TGA) shows that all compounds can be fully
oxidized while retaining the pyrochlore structure, yielding oxygen rich
pyrochlores as white powders. Powder neutron diffraction of Y2TiNbO7-based
samples was done. Refinement of the data for oxygen deficient Y2TiNbO6.76
indicates the presence of a distribution of oxygen over the 8b and 48f sites.
Refinement of the data for oxygen rich Y2TiNbO7.5 shows these sites to be
completely filled, with an additional half filling of the 8a site. The magnetic
and TGA data strongly suggest a preference for a Ti3+/(Nb,Ta)5+ combination, as
opposed to Ti4+/(Nb,Ta)4+, in this pyrochlore family. In addition, the evidence
clearly points to Ti3+ as the source of the localized moments, with no evidence
for localized Nb4+ moments.Comment: Accepted to Journal of Solid State Chemistr
Carbon Concentration Dependence of the Superconducting Transition Temperature and Structure of MgCxNi3
The crystal structure of the superconductor MgCxNi3 is reported as a function
of carbon concentration determined by powder neutron diffraction. The
single-phase perovskite structure was found in only a narrow range of carbon
content, 0.88 < x < 1.0. The superconducting transition temperature was found
to decrease systematically with decreasing carbon concentration. The
introduction of carbon vacancies has a significant effect on the positions of
the Ni atoms. No evidence for long range magnetic ordering was seen by neutron
diffraction for carbon stoichiometries within the perovskite phase stability
range.Comment: 4 figure
Neutron Scattering Study of Crystal Field Energy Levels and Field Dependence of the Magnetic Order in Superconducting HoNi2B2C
Elastic and inelastic neutron scattering measurements have been carried out
to investigate the magnetic properties of superconducting (Tc~8K) HoNi2B2C. The
inelastic measurements reveal that the lowest two crystal field transitions out
of the ground state occurat 11.28(3) and 16.00(2) meV, while the transition of
4.70(9) meV between these two levels is observed at elevated temperatures. The
temperature dependence of the intensities of these transitions is consistent
with both the ground state and these higher levels being magnetic doublets. The
system becomes magnetically long range ordered below 8K, and since this
ordering energy kTN ~ 0.69meV << 11.28meV the magnetic properties in the
ordered phase are dominated by the ground-state spin dynamics only. The low
temperature structure, which coexists with superconductivity, consists of
ferromagnetic sheets of Ho{3+ moments in the a-b plane, with the sheets coupled
antiferromagnetically along the c-axis. The magnetic state that initially forms
on cooling, however, is dominated by an incommensurate spiral antiferromagnetic
state along the c-axis, with wave vector qc ~0.054 A-1, in which these
ferromagnetic sheets are canted from their low temperature antiparallel
configuration by ~17 deg. The intensity for this spiral state reaches a maximum
near the reentrant superconducting transition at ~5K; the spiral state then
collapses at lower temperature in favor of the commensurate antiferromagnetic
state. We have investigated the field dependence of the magnetic order at and
above this reentrant superconducting transition. Initially the field rotates
the powder particles to align the a-b plane along the field direction,
demonstrating that the moments strongly prefer to lie within this plane due to
the crystal field anisotropy. Upon subsequently increasing the field atComment: RevTex, 7 pages, 11 figures (available upon request); Physica
Specific heat study of the Na(0.3)CoO(2).1.3H(2)O superconductor: influence of the complex chemistry
We report results of specific heat measurements on polycrystalline samples of
the layered superconductor, Na(0.3)CoO(2).1.3H(2)O. The electronic contribution
to the specific heat, gamma, is found to be 12.2 mJ/mol-K2. The feature at the
superconducting transition is rather sharp, becoming broad and strongly
suppressed in an applied magnetic field. The data indicate a residual normal
state electronic specific heat at low temperatures, implying that there is a
sizable population of normal state electrons in the samples even below Tc.
Inhomogeneity in the Na content, to which the superconducting state is
exquisitely sensitive, appears to be the most likely explanation for these
results. These results further indicate that special sample handling is
required for an accurate characterization of the superconducting state in this
material.Comment: publication information adde
Superconducting Properties of MgCNi3 Films
We report the magnetotransport properties of thin polycrystalline films of
the recently discovered non-oxide perovskite superconductor MgCNi3. CNi3
precursor films were deposited onto sapphire substrates and subsequently
exposed to Mg vapor at 700 C. We report transition temperatures (Tc) and
critical field values (Hc2) of MgCNi3 films ranging in thickness from 7.5 nm to
100 nm. Films thicker than ~40 nm have a Tc ~ 8 K, and an upper critical field
Hc2 ~ 14 T, which are both comparable to that of polycrystalline powders. Hall
measurements in the normal state give a carrier density, n =-4.2 x 10^22 cm^-3,
that is approximately 4 times that reported for bulk samples.Comment: submitted to PR
Chemical Instability of the Cobalt Oxyhydrate Superconductor under Ambient Conditions
The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2*1.4H2O is shown
through X-ray diffraction and thermogravimetric studies to be one of a series
of hydrated phases of Na0.3CoO2. Further, it is shown that the material is
exceptionally sensitive to both temperature and humidity near ambient
conditions, easily dehydrating to a non-superconducting lower hydrate. The
observation of this stable lower hydrate with c=13.8 angstroms implies that the
superconductivity turns on in this system between CoO2 layer spacings of 6.9
and 9.9 angstroms at nominally constant chemical doping.Comment: 10 pages and 4 figure
Structural Disorder, Octahedral Coordination, and 2-Dimensional Ferromagnetism in Anhydrous Alums
The crystal structures of the triangular lattice, layered anhydrous alums
KCr(SO4)2, RbCr(SO4)2 and KAl(SO4)2 are characterized by X-ray and neutron
powder diffraction at temperatures between 1.4 and 773 K. The compounds all
crystallize in the space group P-3, with octahedral coordination of the
trivalent cations. In all cases, small amounts of disorder in the stacking of
the triangular layers of corner sharing MO6 octahedra and SO4 tetrahedra is
seen, with the MO6-SO4 network rotated in opposite directions between layers.
The electron diffraction study of KCr(SO4)2 supports this model, which on
average can be taken to imply trigonal prismatic coordination for the M3+ ions;
as was previously reported for the prototype anhydrous alum KAl(SO4)2. The
temperature dependent magnetic susceptibilities for ACr(SO4)2 (A = K,Rb,Cs)
indicate the presence of predominantly ferromagnetic interactions. Low
temperature powder neutron diffraction reveals that the magnetic ordering is
ferromagnetic in-plane, with antiferromagnetic ordering between planes below 3
K.Comment: Accepted to the Journal of Solid State Chemistr
Spin Gaps in Coupled t-J Ladders
Spin gaps in coupled - ladders are investigated by exact
diagonalization of small clusters up to 48 sites. At half-filling, the
numerical results for the triplet excitation spectrum are in very good
agreement with a second order perturbation expansion in term of small
inter-ladder and intra-ladder exchange couplings between rungs
(). The band of local triplet excitations moving
coherently along the ladder (with momenta close to ) is split by the
inter-ladder coupling. For intermediate couplings finite size scaling is used
to estimate the spin gap. In the isotropic infinite 4-chain system (two coupled
ladders) we find a spin gap of , roughly half of the single ladder
spin gap. When the system is hole doped, bonding and anti-bonding bound pairs
of holes can propagate coherently along the chains and the spin gap remains
finite.Comment: 11 pages, 5 figures, uuencoded form of postscript files of figures
and text, LPQTH-94/
The complex superstructure in Mg1-xAlxB2 at x~0.5
Electron diffraction and high resolution microscopy have been performed on
Mg1-xAlxB2 with x~0.5. This composition displays a superstructure with a repeat
period of exactly 2c along the c axis and about 10 nm in the a-b plane. The
superstructure results in ring-shaped superreflections in the diffraction
pattern. Irradiation by a strong electron beam results in a loss of the
superstructure and a decrease of about 1% in the c lattice parameter. In-situ
heating and cooling on the other hand showed that the superstructure is stable
from 100 K to 700 K. Possible origins for the superstructure are proposed
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